S1935 Anorectal Melanoma: A Rare but Potentially Fatal Cause of Rectal Bleeding

Abstract

Introduction: Rectal bleeding is a common complaint with many relatively benign causes including hemorrhoids, anal fissures, and colonic diverticula. However, there are more insidious causes of rectal bleeding that can potentially prove to be fatal without early intervention, including colon cancer. Here, we present one rare but potentially deadly cause of rectal bleeding. Case description/methods: A 63-year-old female presents with 1 month history of recurrent painless hematochezia. Prior to presentation, she had 3 episodes large volume bright red blood per rectum. Screening colonoscopy completed 3 years earlier was negative. On presentation the patient was hemodynamically stable, but severely anemic with hemoglobin of 6 g/dl. Digital rectal exam revealed a large anterior anal mass. Further evaluation with colonoscopy revealed a bleeding 2 cm submucosal mass in the anal canal, extending distal to the dentate line, which bled easily with pressure and fibrovascular in appearance. The mass was too friable to biopsy at the time of colonoscopy. She underwent trans-anal resection of the anal mass. Histopathology revealed nodular mucosal malignant melanoma, epithelioid type. Immunohistochemical studies were positive for SOX-10 and Melan A, and negative for cytokeratin AE, CD45, chromogranin, synaptophysin, BRAF. Whole-body PET scan revealed 1.1 cm perirectal lymph node involvement without evidence of distant metastasis. Follow up MRI revealed six suspicious meso-rectal lymph nodes. Eventually, the patient underwent abdominoperineal resection (APR). Histopathology following APR revealed 3 tumor deposits, with negative surgical margin, 7/16 lymph nodes involvement. Patient is following oncology for adjuvant chemotherapy. Discussion: Anorectal melanoma (ARM) is rare, having fewer than 700 cases reported in the literature. Patients with anorectal melanoma typically present with rectal bleeding, anorectal pain, and change in bowel habits. The gold standard for diagnosis is immunohistochemistry. Treatment strategies include surgery with or without immunotherapy. Unfortunately, recurrence and mortality rate are high, with a 5-year survival rate of less than 20% and a median survival of less than 2 years. In summary, ARM is a rare and aggressive variant of mucosal melanoma which is often associated with diagnosis at advanced stage, high metastatic potential, and high mortality. It is imperative that a high index of suspicion be maintained to mitigate the high burden of late diagnoses and advanced disease.Figure 1.: Figure 1A. 4.3 x 3.5 cm mass anterior to a large sigmoid diverticulum with Hounsfield units of 22-36 (red arrow). Hounsfield units of lipomas range from 30 to 70. Figure 1B. 4 cm pedunculated polyp 20 cm from the anal verge. Figure 1C. Microscopy depicting submucosal mass with mature white adipose tissue (red arrow).